Mosquitoes vs Malathion

Every summer, the city of Winnipeg's insect control branch uses a variety of insecticides – both biological and chemical – to control the population of harmful and nuisance insects, such as mosquitoes, cankerworms, elm bark beetles and other bugs.

The city uses malathion to control adult mosquitos. Malathion is an organophosphate insecticide that has been registered for use in Canada since the 1950s.

For mosquito control, malathion is applied by fogging, using ultra-low volume equipment on trucks. The equipment creates a very fine mist of about 60 grams of malathion per hectare.

Besides mosquito control, malathion is also used to control other insects in agricultural settings, such as food or ornamental crops.

Is it dangerous?

Malathion is toxic to all insects, including those considered beneficial to humans, such as honeybees. It is also toxic to fish and other aquatic animals. Health Canada's Pest Management Regulatory Agency says the insecticide "displays low toxicity" to birds and mammals.

The chemical degrades rapidly in the environment; officials say it has a half-life of less than one day in soil, which means it takes less than a day for half of a dose to disappear. In the air, its half-life is 1.5 days, on plants about 5.5 days, and in water half a day to 19 days.

Opponents to malathion say the chemical can weaken the immune system and cause cancer, birth defects, intestinal disorders, kidney problems and other health problems.

However, the Pest Management Regulatory Agency (PMRA) recently completed a re-evaluation of public health uses of malathion. It determined the insecticide is still "acceptable for use in controlling adult mosquitoes."

Even for someone who is outside during fogging, the PMRA says, exposure levels would be much lower than those that caused problems in animal tests.

Young animals were more sensitive to the toxic effects of malathion, and the chemical appeared to have adverse effects on the offspring of test animals.

However, the PMRA says the amount of malathion ingested by those animals is far greater than the amount people would be exposed to in fogging situations.

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What to do when fogging is underway

The City of Winnipeg informs the public when its mosquito-fogging program is underway. Residents can register their property on a do-not-spray list – the city leaves a 100-metre buffer around a registered property.

Hundreds of people in Winnipeg request exemptions from the insect-control programs each year. In the Wolseley neighbourhood, for the past several years, enough residents have registered in opposition that the entire neighbourhood has been left out of the insect-control program.

Residents can ask also to receive an email or phone call with detailed information about when and where the city will fog eight hours before the procedure begins.

Whenever possible, remain indoors during and immediately after spraying.

Close all windows and doors. Turn off air conditioning units and close vents to circulate indoor air before spraying begins.

Cover swimming pool surfaces when it is feasible.

Cover outdoor furniture and play equipment or rinse them off with water after spraying is finished.

Wash homegrown fruits and vegetables with water before cooking or eating them.

Cover ornamental fish ponds to avoid direct exposure.

If direct contact with malathion spray is unavoidable, the PMRA recommends people protect their eyes, wash exposed skin, and wash clothing that comes in contact with the spray separately from other laundry.